Image sensors are devices which receive an optical signal from an object and convert the optical signal to an electrical signal. The electrical signal can then be transmitted for further processing, such as digitization and then storage in a storage device such as a memory or optical or magnetic disk, or for presentation on a display, printing, etc. Image sensors are typically used in devices such as digital cameras, camcorders, printers, facsimile machines, etc.
Image sensors are typically of two types, namely, charge coupled device (CCD) sensors and CMOS image sensors (CIS). CCD sensors typically have advantages including low noise operation and device uniformity. CIS devices are typically characterized by low power consumption and can be operated at high speed due to a high frame rate capability.
FIG. 1 is a schematic cross-sectional diagram of a conventional image sensor. Referring to FIG. 1, the image sensor includes a substrate 1. An isolation layer 3 is formed in the substrate 1. An n-type photodiode 5 is formed in the substrate 1 by an n-type high-energy ion implantation using photoresist as a mask. A transfer gate structure 10, which includes a gate dielectric 7 and a gate electrode 9 made of, for example, polysilicon, is formed over the substrate 1. An n-type floating diffusion region 13 is formed in the substrate 1 by n-type high-concentration ion implantation using the transfer gate structure 10 as an implantation mask. A p-type hole accumulated device (HAD) region 15 is formed by p-type high-concentration ion implantation using the transfer gate structure 10 as an implantation mask. A channel region 17 is formed between the floating diffusion region 13 and the HAD region 15.
As illustrated in FIG. 1, the n-type floating diffusion region 13 and the p-type HAD region 15 are formed to be self-aligned with the transfer gate structure 10. However, because of possible misalignment, the n-type photodiode 5 may not extend laterally to the edge of the transfer gate structure 10. This results in formation of an offset region 19 between the edge of the photodiode 5 and the edge of the transfer gate structure 10. This offset region results in an undesirable “image lag” phenomenon in the sensor device.
FIG. 2 is an energy band diagram corresponding to the image sensor device of FIG. 1. Referring to FIG. 2, the diagram shows the energy bands for the photodiode region 5, the floating diffusion region 13, the channel region 17 and the offset region 19 of the device. FIG. 2 illustrates potential level of a corresponding conduction band EC in the photodiode 5, offset 19, channel 17 and floating diffusion 13 regions.
As illustrated in FIG. 2, some amount of electron charges E1 and E2 are generated in the photodiode region 5 in response to incident light. The amount of charge is dependent upon the intensity of the incident light. When the transfer gate electrode 9 has a high pulse applied, the potential level indicated in FIG. 2 is lower, i.e., more positive. That is, ECH′>ECH″. The E1 group of electron charges will be transferred to the floating diffusion region 13. However, the E2 group of charges will remain in the photodiode region. These remaining E2 charges are trapped by the high potential barrier HB caused by the offset region 19. The E2 group of charges remaining in the photodiode region result in the image lag phenomenon.